THE EMBRYOLOGY OF THE OPOSSUM 21 



Also of interest in this connection is the evidence of a dif- 

 ference in fertility between the matings which occur during 

 daylight hours and those which occur in the night or very 

 early morning. Selenka said that he had never seen mating 

 occur before 7 in the morning or after 11. Though I have 

 seen one mating at 3 o'clock in the afternoon, it is true that 

 it is very exceptional for matings to occur after 11 A.M. On 

 the other hand, the 7 o'clock early limit is quite definitely 

 wrong. By far the majority of all matings occur before that 

 time, and I have some evidence that the matings which occur 

 between 7 and 11 are not really normal. 



Of the sixty-one matings which have been observed in my 

 colony after 7 o 'clock in the morning, only eleven were fertile. 

 There is thus a ratio of five and one-half infertile matings 

 to one fertile mating after 7 A.M. 



Only occasionally have observations been continued through 

 the whole 24 hours, so the total number of matings which oc- 

 curred before 7 A.M. is not known for any large number of 

 days. But the total number of adult females which were in 

 the colony at any given time is known, and the normal length 

 of the oestrous cycle (Hartman, '23) is known, as already 

 mentioned. If it is assumed for the sake of a calculation that 

 every adult female mated once each cycle except during the 

 annual anoestrous period and the times when she was either 

 pregnant or lactating, the maximum possible number of 

 matings in my colony can be computed for any given length 

 of time. For the year 1935 this number is 209. Since thirty- 

 one matings were observed after 7 o'clock in the morning, 

 there are left only 178 conceivably possible matings which 

 might have occurred before seven. If the total number of 

 litters born during 1935, minus those from matings which 

 occurred after 7 o'clock, be multiplied by 54, the product 

 in 203.5. This is the number of matings which must have 

 occurred before 7, if the ratio of infertile matings is as high 

 before 7 as it is afterward. But this number is 13% greater 

 than 178, the maximum possible number of matings before 

 7 o'clock. In other words, it is impossible that the percentage 



